This invention relates to precipitation-hardenable martensitic stainless steels and in particular to a precipitation-hardenable martensitic stainless steel that provides a unique combination of machinability, processability, and toughness.
The known precipitation-hardenable stainless steels provide high hardness and strength through an age-hardening heat treatment in which a strengthening phase is formed in the relatively, more ductile matrix of the alloy. Such alloys have been used principally in components for aerospace applications. Another type of stainless steel that is designed to provide relatively high strength is the so-called xe2x80x9cstraightxe2x80x9d martensitic stainless steel. An example of such a steel is AISI Type 416 alloy. Such steels achieve high strength when they are quenched from a solution or austenitizing temperature and then tempered. Although there are free-machining grades of the straight martensitic stainless steels, there has not been any known martensitic precipitation-hardenable stainless steel that could be classified as a truly xe2x80x9cfree-machiningxe2x80x9d grade. In other words, none of the known grades of precipitation-hardenable martensitic stainless steels contain more than about 0.15% of a free-machining additive such as sulfur or selenium. Because of the simplicity of heat treating the precipitation-hardenable martensitic stainless steels compared to the straight martensitic stainless steels, it would be desirable to have a precipitation-hardenable martensitic stainless steel that provides true free-machining capability.
Hitherto, attempts have been made to produce martensitic precipitation-hardenable stainless steels that provide xe2x80x9cenhanced machinabilityxe2x80x9d relative to the standard grades. Such attempts have included the use of limited amounts of free-machining additives such as sulfur or selenium. Alloys have been described that may contain up to relatively high amounts of such additives, e.g., up to 0.40 weight percent, up to 0.5 weight percent, or up to 0.15 weight percent of sulfur or selenium. However, there has not been a commercially produced precipitation-hardenable martensitic stainless steel that actually contains more than about 0.036 weight percent of sulfur or selenium.
The principal reason for the unavailability of a true free-machining precipitation-hardenable martensitic stainless steel is that the presence of the usual free-machining additives such as sulfur and selenium adversely affects important properties of the precipitation-hardenable grades of stainless steels. For example, the presence of sulfur in a known grade of precipitation-hardenable stainless steel has resulted in poor processability, such that the steel tears or splits during hot working or cracks during cold processing or quenching. Also, the presence of sulfur adversely affects the toughness and ductility of the alloy.
In accordance with the present invention, there is provided a free-machining, precipitation-hardenable martensitic stainless steel, having a unique combination of machinability, processability, and toughness. The broad, intermediate, and preferred compositional ranges of the steel alloy of the present invention are as follows, in weight percent:
The balance of the alloy is essentially iron, except for the usual impurities found in commercial grades of martensitic, precipitation-hardenable stainless steels and trace amounts of other elements which may vary from a few thousandths of a percent up to larger amounts that do not objectionably detract from the desired combination of properties.
The foregoing tabulation is provided as a convenient summary and is not intended to restrict the lower and upper values of the ranges of the individual elements of the alloy of this invention for use in combination with each other, or to restrict the ranges of the elements for use solely in combination with each other. Thus, one or more of the element ranges of the broad composition can be used with one or more of the other ranges for the remaining elements in the preferred composition. In addition, a minimum or maximum for an element of one preferred embodiment can be used with the maximum or minimum for that element from another preferred embodiment. Throughout this application, the term xe2x80x9cpercentxe2x80x9d or the symbol xe2x80x9c%xe2x80x9d means percent by weight, unless otherwise indicated.
The precipitation hardenable alloy according to this invention contains at least about 14.0% and preferably at least about 14.5% chromium in order to provide the desired level of corrosion resistance. Too much chromium promotes the formation of an undesirable amount of ferrite in this alloy, which adversely affects the toughness and ductility provided by the alloy. Accordingly, the alloy contains not more than about 15.5% and preferably not more than about 15.0% chromium.
Sulfur benefits the machinability of this alloy and at least about 0.15%, preferably at least about 0.17%, sulfur is present in order to obtain a significant improvement in machinability, particularly form-tool machinability. The alloy contains not more than about 0.35%, better yet not more than about 0.30%, and preferably not more than about 0.25% sulfur because too much sulfur adversely affects the processability, toughness, and the corrosion resistance of this alloy.
Nickel promotes the formation of austenite when the alloy is heated at an elevated temperature so that the alloy will readily form martensite during quenching from the elevated temperature. Nickel also contributes to corrosion resistance and toughness in this alloy. Good toughness is important not only for cold processability, but also to inhibit cracking of the alloy when it is quenched, a problem that typically arises in stainless steels containing elevated amounts of sulfur. Nickel also promotes the formation of reverted austenite during the age-hardening process. The presence of a limited amount of reverted austenite in the alloy is beneficial to the toughness of the alloy. For these reasons, the alloy according to this invention contains at least about 5.0% nickel.
Excessive nickel depresses the martensite transformation temperature, which leads to retained austenite after the alloy is quenched. The presence of retained austenite adversely affects the strength capability of the alloy. Therefore, the alloy contains not more than about 6.0% nickel and preferably not more than about 5.5% nickel.
Molybdenum contributes to the corrosion resistance of the alloy, particularly resistance to pitting-type corrosion. Molybdenum also benefits the toughness and ductility provided by this alloy. Accordingly, the alloy contains at least bout 0.50%, and preferably at least about 0.70% molybdenum. Molybdenum promotes the formation of ferrite, too much of which, as noted above, adversely affects the toughness and ductility of this alloy. Therefore, the alloy contains not more than about 1.2% and preferably not more than about 1.0% molybdenum.
At least about 3.0%, preferably at least about 3.2%, copper is present in this alloy as a precipitation hardening agent. During the age hardening heat treatment, the alloy achieves substantial strengthening through the precipitation of fine, copper-rich particles from the martensitic matrix. Too much copper adversely affects the hot workability of the alloy. Therefore, the alloy contains not more than about 4.0% and preferably not more than about 3.8% copper.
At least about 0.10% niobium is present in this alloy primarily as a stabilizing agent against the formation of chromium carbonitrides which are deleterious to the corrosion resistance of the alloy. Too much niobium causes excessive formation of niobium carbides, niobium nitrides, and/or niobium carbonitrides which adversely affect the good machinability provided by this alloy. Too many niobium carbonitrides also adversely affect the alloy""s toughness. Furthermore, excessive niobium results in the formation of an undesirable amount of ferrite in this alloy. Therefore, the alloy contains not more than about 0.30%, better yet not more than about 0.25%, and preferably not more than about 0.20% niobium. Those skilled in the art will recognize that tantalum may be substituted for some of the niobium on a weight percent basis. However, tantalum is preferably restricted to not more than about 0.05% in this alloy.
A small but effective amount of boron may be present in amounts up to about 0.010%, preferably up to about 0.005%, to benefit the hot workability and toughness of this alloy.
The balance of the alloy composition is iron except for the usual impurities found in commercial grades of martensitic precipitation-hardenable stainless steels intended for similar use or service. For example, the interstitial elements carbon and nitrogen are restricted to low levels in this alloy in order to benefit the machinability and processability of the alloy, especially during cold processing and quenching. Therefore, the alloy contains not more than about 0.030%, better yet, not more than about 0.025%, and preferably not more than about 0.020% of each of carbon and nitrogen. Other elements such as manganese, silicon, and phosphorus are also maintained at low levels because they adversely affect the good toughness provided by this alloy. More specifically, this alloy contains not more than about 0.75% and preferably not more than about 0.50% manganese because manganese combines with sulfur to form manganese sulfides which adversely affect the corrosion resistance of the alloy. Silicon is typically added to provide deoxidation of the alloy during refining. However, silicon promotes the formation of ferrite in this alloy. Therefore, the alloy contains not more than about 0.75% and preferably not more than about 0.50% silicon. This alloy contains not more than about 0.040%, better yet, not more than about 0.035%, and preferably not more than about 0.030% phosphorus because it adversely affects the toughness and the machinability of this alloy.
The alloy according to this invention is preferably arc-melted in air (ARC), but can also be melted by vacuum induction melting (VIM). The alloy can be refined by vacuum arc remelting (VAR). The alloy may be produced in various product forms including billet, bar, rod, and wire. The alloy is preferably hot worked from a temperature of about 2150-2350xc2x0 F. The alloy is solution treated by heating at about 1800-2000xc2x0 F. for about one-half to one hour and then rapidly quenched, preferably with water. The alloy is then aged to final strength by heating at about 900-1150xc2x0 F. for up to about 4 hours, followed by cooling in air. The alloy may be used to fabricate a variety of machined, corrosion resistant parts that require high strength and good toughness. Among such end products are valve parts, fittings, fasteners, shafts, gears, combustion engine parts, components for chemical processing equipment and paper mill equipment, and components for aircraft and nuclear reactors.